Unary Phase Diagram
Unary systems are single component systems. That is why we can not take composition as a variable. Yes, unary diagrams are equilibrium diagrams but for these systems we have to take temperature and pressure as variables for the equilibrium conditions.
We take temperature along the ordinate (y-axis) and the pressure along the abcissa (x-axis).
Since there are two external variables (i.e T & p), we use generalized form of Gibb's phase rule for unary systems ( i.e. P + F = C + 2 ).
Eg. Unary diagram for pure iron is shown in the figure. There are total 5 phases of iron shown, i.e. α-Fe, γ-Fe, Hexaferrum, δ-Fe and Liquid.
- α-Fe = BCC
- γ-Fe = FCC
- δ-Fe = BCC
- Hexaferrum = HCP
For any single phase region, if we apply the phase rule : C=1, P=1, and thus, F=2, i.e. there are two degrees of freedom. We can vary both temperature and pressure while still being in the single phase region. This situation is called bivariant equilibrium.
For any two phase coexistence line, C=1, P=2, and thus, F=1, i.e. we can only vary one of the two variables and other will be automatically fixed. This situation is called univariant equilibrium.
At triple point, where three phases coexist, C=1, P=3, and thus, F=0, i.e. we can not change any of the variable. Because if we change either of temperature and pressure, one or two of the phases will disappear. Thus three phase equilibrium exists only for a fixed value of temperature and pressure.
This situation is called invariant equilibrium.
Quick Note :
- Single phase equilibrium is characterised by an area.
- Two phase equilibrium is exists along a line.
- Three phase equilibrium is only possible at a fixed point.
The maximum number of phases that can coexist in unary system is 3.